1. Field of the Invention
The present invention relates generally to the wet process manufacture of phosphoric acid, and more particularly, to chemical aids for wet-grinding phosphate rock.
1. Description of the Prior Art
In the wet process manufacture of phosphoric acid, phosphate-containing ore is mined and slurried in water. The ore is upgraded by washing with water to remove undesirable gangue material which consists mainly of fine clay. This is achieved by the use of cyclones and flotation equipment commonly used in the beneficiation of ores. The washed phosphate rock, sometimes called "pebble phosphate" is then shipped to the phosphoric acid plant for further processing.
The beneficiated phosphate rock received from the mine is, in most cases, too coarse and must be milled to a smaller particle size. Typical particle size distributions for various sources of Florida rock are 8-20%+35 mesh, 59-94%+60 mesh, and 88-95%+100 mesh. The phosphate rock is slurried in water (typically 65-70% solids) and ground in a ball mill to 0.5-3%+35 mesh and 25-60%-200 mesh. The optimum size distribution depends on the particular reactor design used for digestion of the rock, and is chosen to maximize conversion to phosphoric acid.
The ground phosphate rock slurry is combined with recycled phosphoric acid, sulfuric acid, and water in a reactor heated to 165.degree.-190.degree. F., and the mixture is vigorously agitated for 2-8 hours. The phosphate is converted to phosphoric acid and calcium sulfate dihydrate, gypsum, (or in the higher pressure process, hemihydrate) precipitates as a byproduct. The newly-formed gypsum, together with any unreacted phosphate rock, are removed from the crude phosphoric acid by filtration. The filtrate typically contains 25-30% P.sub.2 O.sub.5 (dihydrate process) or 40% (hemihydrate process) is retained while the washed filter cake is discharged as waste. The acid filtrate is finally concentrated stage-wise in vacuum evaporators to 54% to P.sub.2 O.sub.5 (100% H.sub.3 PO.sub.4) or to as high as 72% P.sub.2 O.sub.5 (super phosphoric acid).
In its present form, wet-grinding of phosphate rock was developed in 1973 and is practiced in almost all U.S. Plants. The advantages of wet-grinding over dry-grinding are:
1. elimination of energy costs for drying rock; PA1 3. reduction in the size of grinding mills and associated energy consumption. PA1 1. it is very difficult to control the amount of oversize rock (even when operated in a closed-loop grinding process), this leads to incomplete extraction of the phosphate value from the ore; PA1 2. reduced P.sub.2 O.sub.5 strength in the reactor due to dilution with water associated with the slurry, this requires larger evaporators, and higher evaporation costs; PA1 3. reduced volume of water available for washing the gypsum filter cake, resulting in higher water-soluble P.sub.2 O.sub.5 losses; and PA1 4. overgrinding of soft, porous rock which produces slurries with high viscosity and a rock which is too fine and hyperactive, a hyperactive rock yields very fine gypsum crystals which are difficult to filter. PA1 1. better grinding by reducing the proportion of oversized particles and more efficient operation of cyclones used in the closed-loop arrangement; this results in improved extraction of the phosphate (lower insoluble losses); PA1 2. a higher percent solids in the slurry resulting in less water to evaporate from the phosphoric acid and more water available for washing the filter cake (lower water-soluble P.sub.2 O.sub.5 losses); and PA1 3. lower residence time in the ball mill without sacrificing percent solids and particle size distribution in the phosphate rock slurry; this translates into higher throughput and lower energy consumption.
2. elimination of pollution control (mainly phosphate rock dust); and
The major disadvantages of wet-grinding are:
The use of the appropriate grinding aid alleviates the aforementioned problems associated with the wet-grinding of phosphate rock. This is possible due to the significant reduction in slurry viscosity afforded by the grinding aid which allows:
The present invention advantageously provides a grinding aid which provides the above-listed benefits. An additional benefit of the invention, not related to grinding, is the ability of the grinding aid to reduce or eliminate foam during digestion of the phosphate rock. Presently, most, if not all, phosphoric acid plants which process uncalcined rock, must use a defoamer to prevent formation of a stable dense, foam. This foam is generated during the release of carbon dioxide, hydrogen fluoride, and silicon tetrafluoride produced by the action of sulfuric acid on carbonate and fluoride compounds present in the phosphate rock. The present invention advantageously also acts as a defoamer.